1. Field of the Invention
The present invention relates to radio apparatuses, and it particularly relates to a radio apparatus using multiple subcarriers and a communication system utilizing said radio apparatus.
2. Description of the Related Art
An OFDM (Orthogonal Frequency Division Multiplexing) modulation scheme is one of multicarrier communication schemes that can realize the high-speed data transmission and are robust in the multipath environment. This OFDM modulation scheme has been applied to the wireless standards such as IEEE802.11a/g and HIPERLAN/2. The packet signals in such a wireless LAN are generally transferred via a time-varying channel environment and are also subjected to the effect of frequency selective fading. Hence, a receiving apparatus generally carries out the channel estimation dynamically.
In order for the receiving apparatus to carry out the channel estimation, two kinds of known signals are provided within a packet signal. One is the known signal, provided for all carriers in the beginning of the packet signal, which is the so-called preamble or training signal. The other is the known signal, provided for part of carriers in the data area of the packet signal, which is the so-called pilot signal (See Reference (1) in the following Related Art List, for instance).
Related Art List
    (1) Sinem Coleri, Mustafa Ergen, Anuj Puri and Ahmad Bahai, “Channel Estimation Techniques Based on Pilot Arrangement in OFDM Systems”, IEEE Transactions on broadcasting, vol. 48, No.3, pp. 223-229, September 2002.
In wireless communications, adaptive array antenna technology is one of the technologies to realize the effective utilization of frequency resources. In adaptive array antenna technology, the directional patterns of antennas are controlled by controlling the amplitude and phase of signals, to be processed, in a plurality of antennas, respectively. One of techniques to realize higher data transmission rates by using such an adaptive array antenna technology is the MIMO (Multiple-Input Multiple-Output) system. In this MIMO system, a transmitting apparatus and a receiving apparatus are each equipped with a plurality of antennas, and a plurality of packet signals to be transmitted in parallel are set (hereinafter, each of data to be transmitted in parallel in the packet signal is called “stream”). That is, streams up to the maximum number of antennas are set for the communications between the transmitting apparatus and the receiving apparatus so as to improve the data transmission rates.
Moreover, combining such a MIMO system with the OFDM modulation scheme results in a higher data transmission rate. In such a MIMO system, CSMA (Carrier Sense Multiple Access) is carried out to allow the base station apparatus to multiplex a plurality of terminal apparatuses. For the purpose of improving the transmission efficiency or reducing the processing delay, the base station apparatus specifies, in partial periods of time, the timing at which the signals are to be transmitted to a plurality of terminal apparatuses (hereinafter referred to as “transmit timing”) and the timing at which the signals from a plurality of terminal apparatuses are to be received (hereinafter referred to as “receive timing”). Then the base station apparatus informs respectively the plurality of terminal apparatuses of said specification, and each of the plurality of terminal apparatuses carries out a processing in accordance with said specification (hereinafter, such a processing will be referred to as “assignment mode”). Here it is assumed that after a plurality of transmit timings for the plurality of terminals are specified consecutively, a plurality of transmit timings are specified consecutively. A terminal apparatus receives a signal at the specified transmit timing. When the receiving has been successful, a terminal apparatus generates an ACK signal and transmits the ACK signal to the base station apparatus at the specified receive timing. When the receiving has failed, the terminal apparatus does not generate the signal.
To raise the transmission efficiency of such a MIMO system, the longer packet signal length is preferred. Thus, when transmitting the data to a receiving apparatus, a transmitting apparatus gathers up the data to be transmitted and then generates packet signals. In this manner, the aggregation of packet signals may be done also in a case when they are destined to a single terminal apparatus, besides the case when they are destined to a plurality of terminal apparatuses. The aggregated packets destined to a single terminal apparatus and those destined to a plurality of terminals may be processed without making distinction there between. However, in such a case, if the aggregated packets destined to a single terminal apparatus undergo the above assignment mode, it will be wasteful for both the base station apparatus and the terminal apparatuses, thus causing the transmission efficiency to deteriorate.